This invention relates to a method for traction control for use in automotive vehicles wherein wheel rotation is continuously monitored. The wheel slip is determined by comparing the rotation of the driven wheels with the rotation of the non-driven wheels and/or a reference quantity. The driving slip is controlled by a braking action on the wheel tending to spin up. Brake systems for implementing this method are embraced by the present invention.
Methods and apparatus for traction control, that is, for preventing an undesired spin-up of the driven vehicle wheels as a result of an excessive driving torque or adverse road conditions, are known and have been provided in a variety of configurations. In many systems of this type, the vehicle wheels are equipped with sensors, for example, inductive pick-ups, furnishing an electrical signal whose frequency is dependent on the rotation of the wheel. An incipient spin-up condition is detectable by comparing the rotational velocity of a driven wheel with the rotational velocity of the non-driven wheel on the same side of the vehicle or with a reference quantity and by evaluating abrupt changes in the wheel rotation. In these cases, the metering out of brake pressure applied to the wheel brake of the wheel concerned will retard the wheel, whereby the traction or the positive wheel slip is limited to the desired value. In order to prevent the brake from being overloaded, the traction control further operates on the engine, reducing the engine torque by automatically reducing the supply of fuel, by changing the position of the throttle valve, etc. (German patent specifications DE-PS Nos. 3,127,302, 3,140,959). Also, it is known to have the traction control act upon the differential which is accomplished by engagement of the differential pawl when a tendency to spin is established (German patent specification DE-PS No. 3,342,574).
The complexity of wheel sensors, circuits for logically combining and evaluating the sensor signals as well as for the generation of control signals and in particular for the metered application of brake pressure from an auxiliary pressure source is considerable. Therefore, an obvious solution is to combine traction control apparatus of this type with electronically controlled anti-lock brake systems because these are equipped with the major part of the components required for traction control, particularly with wheel sensors, electronic circuits and brake pressure control valves. The additional provisions required for the metering out of brake pressure during the traction or driving slip control are limited to an extended logic and a few brake pressure control valves.
However, the intervention in the engine, that is, the throttling of the driving torque to a value optimum for the traction, causes difficulties and is a considerably more involved procedure. For the torque to be controlled independent of the position of the accelerator pedal, it is necessary to substitute a so-called electronic accelerator pedal for the accelerator pedal conventionally mechanically linked with the carburetor or injection apparatus. Also, the functional reliability of these components has to be very high because a delayed or insufficient reduction of the torque could overload and damage the brake. Further, an insufficient torque because of insufficient traction control is unacceptable, since conditions may occur in which a high traction or a high acceleration of the vehicle is absolutely mandatory to avoid dangerous situations.
It is, therefore, an object of this invention to provide a method for controlling the traction or the starting and driving slip which can be implemented with relatively little effort, but which operates reliably and permits an overload condition of the brake system to be avoided. By virtue of its relatively low manufacturing expenditure, the system is particularly suitable for vehicles in the medium and lower power ranges.